One prior art stereo FM radio receiver having an automatic frequency control circuit is introduced hereunder, referring to FIG. 1. Reference numeral 1 designates an antenna, 2 is an FM tuner, 3 is an intermediate frequency (IF) amplifying and detecting circuit, 4 is a noise canceller, and 5 is a multiplexer connected to an audio stage via paths 6. A terminal X.sub.1 of the IF amplifier/detector 3 is connected to an intermediate signal meter (not shown) for indicating the level of a received signal. An output voltage Vs from the terminal X.sub.1 for driving the signal meter is applied to a terminal X.sub.2 of the multiplexer 5 via a resistor R.sub.1 so as to reduce noises when received signals are weak during stereophonic reception. The same output voltage Vs is also applied to a terminal X.sub.3 of the same multiplexer 5 after being divided by resistors R.sub.2 and R.sub.3 so as to reduce noises when received signals are weak during stereophonic or monophonic reception. The voltage Vs appearing at the terminal X.sub.1 increases with the input level at the antenna 1 up to a saturation as shown in FIG. 2. Thus the stereo FM receiver reduces noises in a weak FM reception, by entering the voltage Vs in the multiplexer 5 to actuate it to effect two controls herein named "stereo noise control (SNC)" and "high-cut control (HCC)", respectively.
It should be noted that since the circuit of FIG. 1 is known and not directly related to the invention, a detailed explanation thereof is omitted here.
The stereo noise control (SNC) is first discussed hereunder. It is known that signal-to-noise ratios in stereophonic reception and monophonic reception represent changes as shown by the coordinate system of FIG. 3, in which the abscissa is the signal level received by the antenna 1 (FIG. 1), the ordinate is the signal-to-noise ratio at the tuner 2 in decibel, a line 7 shows changes in signal-to-noise ratio in stereophonic reception, and a line 8 shows changes in signal-to-noise ratio in monophonic reception. In practice, the two lines 7 and 8 do not maintain a parallel relationship due to the hum level, carrier leakage and other factors. More specifically, the difference between the two lines 7 and 8 is small as shown by numeral 9 at their horizontal portions (saturations) designated by 10 and 11, but it is large as shown by numeral 9' at their angled portions. This means that the difference between the signal-to-noise ratios in stereophonic and monophonic receptions is large at low antenna input levels but small at high antenna input levels. For example, the difference is about 21.6 dB when the time constant for emphasis is 50 .mu.sec and about 23 dB when the time constant is 7.5 .mu.sec. Therefore, at a point A (FIG. 3) where the antenna input level is rather low, a large noise is heard by human ears in stereophonic reception.
In this connection, a voltage from the terminal X.sub.1 (FIG. 1) is applied to the terminal X.sub.2 of the multiplexer 5 so that the multiplexer 5 automatically changes its separation ratio in response to the voltage and with changes in the antenna input level from an intermediate level to a low level, thereby reducing the noise sensed by human ears. This is what the Applicant calls "SNC" in the instant text. FIG. 4 shows how the separation ratio is changed with the voltage Ve applied to the terminal X.sub.2.
Next, the high-cut control (HCC) is discussed below. When received signals are weak in either stereophonic or monophonic FM reception, noises are conspicuous in some reproduction frequency bands. This is because an FM broadcasting uses a wide bandwidth, and the received signals are often not reproduced in a good condition in some frequencies near the definite limits of the bandwidth when in particular the received signals are weak, so that noises are stressed with respect to the reproduced signals and deteriorate the signal-to-noise ratio. In order to reduce the deterioration in signal-to-noise ratio, the frequency characteristics of the multiplexer 5 are automatically controlled as the antenna input level changes from an intermediate level to a low level. FIG. 4 shows how the control is effected. A graph 12 shows how the attenuation is changed with fequency when the voltage applied to the terminal X.sub.3 is V.sub.x31 which is responsive to the saturation value of the voltage Vs of the terminal X.sub.1 corresponding to an intermediate antenna input level. Similarly, a graph 13 shows how the attenuation is changed when the terminal X.sub.3 receives a voltage V.sub.x32 which is smaller than the voltage V.sub.x31 and corresponding to a smaller value of the voltage Vs at the terminal X.sub.1 responsive to a lower antenna input level. A graph 14 corresponds to a voltage V.sub.x33 at the terminal X.sub.3 which is smaller than V.sub.x32 and responsive to a still smaller voltage at the terminal X.sub.1 with a still lower antenna input level. In a high reproduction frquency band, the graphs 12, 13 and 14 drop with different curves. Namely, the graph 13 represents a larger drop than the graph 12, and the graph 14 represents a still larger drop than the graph 13. This means that the attenuation is increased with decrease of the voltage at the terminal X.sub.3, i.e. with decrease of the antenna input level. In this manner, the signal-to noise ratio in a low antenna input level is effectively improved. This is what the Applicant calls "HCC" in the present text.
In general the SNC and HCC are performed when the voltage at the terminal X.sub.2 is 1.5 V to 0 V.
Noises are also produced due to multipaths in either stereophonic or monophonic FM reception. FIG. 6 is a block diagram of a prior art FM stereo radio receiver which is designed to reduce such noises.
Reference numerals 15 and 19 denote buffer circuits, 16 is an ac component detector, 17 is a rectifier, 18 is a smoothing circuit, and the other reference numerals designate the same circuit components as those in FIG. 1. The ac components in the voltage Vs at the terminal X.sub.1 are detected by the ac component detector 16 and rectified by the rectifier 17 in a polarity opposite to the original dc components of the voltage Vs. The rectified current is smoothed by the smoothing circuit 18 and combined at a point Y with the original dc components of the voltage Vx supplied via the buffer circuit 15. The voltage at the point Y is applied to the terminals X.sub.2 and X.sub.3 of the multiplexer 5 via the buffer circuit 19. Since the voltage at the point Y becomes such that the original dc components are forcibly levelled down by the rectified and smoothed components, the high-cut control (HCC) and stereo noise control (SNC) are activated to effectively improve the signal-to-noise ratio during intermediate and low levels of the antenna input.
In the prior art devices, however, since the HCC and SNC are governed simply by the output voltage from the signal meter terminal X.sub.1 of the IF amplifier/detector 3, their control operations are uniformly, inflexibly determined by resistors R.sub.1, R.sub.2 and R.sub.3 (FIG. 1).
Therefore, the prior art control circuits produce the following drawbacks due to varieties in characteristics of receivers. Assuming that the voltage at the terminal X.sub.1 of the IF amplifier/detector 3 is equal in some receivers, the signal-to-noise ratio of one receiver as a whole or at the IF amplification stage is often different from that of another receiver. This means that selection of the resistors R.sub.1, R.sub.2 and R.sub.3 simply depending on the antenna input level does not always produce the best result.
Beside this, while the antenna input level is low, the noise canceller 4 is often erroneously activated and produces crackles or other noises in particular in high frequencies and high modulation degrees. The prior art control devices, however, has no means to reduce these noises.